Some internal combustion engines utilize a fuel delivery system that enables direct injection of fuel into one or more cylinders of the engine. Direct injection engines may be operated across a broad range of ambient conditions, including relatively cold temperatures. However, because directly injected fuel receives less heat energy during the intake process, as compared with port injection for example, during cold start or engine warm-up conditions, evaporation of directly-injected fuel into the cylinder may be reduced or may not occur before a combustion event.
In one approach, to address the reduction in evaporation at colder temperatures, an excess amount of fuel may be directly-injected into the cylinder so that the fuel that evaporates provides an air/fuel ratio that is near stoichiometric or other suitable ratio. After combustion, the excess fuel in the cylinder that did not participate in the combustion process may be exhausted during the exhaust stroke as hydrocarbon (HC) emissions. Thus, emissions may be increased and/or fuel efficiency may be decreased during these and other conditions.
In another approach, as described in U.S. Pat. Nos. 4,641,613 and 5,657,730, the fuel supply system may be stopped during an operation where air within the cylinder is compressed over one or more cycles while intake and/or exhaust valves or throttles are closed, thereby increasing the temperature of the air within the cylinder. When the air is heated to a suitable temperature by the compression operation, the injection of fuel can be initiated to cause combustion of the air and fuel mixture.
The inventors herein have recognized a disadvantage with these approaches. Specifically, the heating of the air within the combustion chamber in this manner may provide insufficient evaporation of later injected fuel due to the additional time that may be needed to transfer heat energy from the warmed air to the injected fuel. In other words, the direct injection of fuel after the air within the cylinder is heated may still not provide the desired air/fuel ratio depending on the rate of evaporation. Thus, the above approaches may still use additional fueling of the cylinder to achieve a suitable air/fuel ratio.
In another approach as disclosed herein, the above issues may be addressed by a method of operating an engine including at least one cylinder and a piston disposed within the cylinder, the method comprising during a first condition, injecting fuel into the cylinder and subsequently operating the piston to perform one compression stroke before combusting the injected fuel; and during a second condition, injecting fuel into the cylinder and subsequently operating the piston to perform at least two compression strokes before combusting the injected fuel.
In this manner, evaporation of the fuel within the cylinder may be selectively increased since the fuel may be heated and at least partially evaporated at least during each of the compression strokes. For example, during a compression stroke, the charge temperature and/or enthalpy of the charge rises, which allows the injected fuel to be at least partially evaporated during the first compression. Then, at least some of the evaporated fuel may remain in the evaporated state during the expansion stroke. As compression is performed again, still more evaporation of fuel can be achieved. Depending on the amount of evaporation desired, the number of compressions may be adjusted, thereby achieving improved starting emissions, for example.
Note that while direct injection of fuel may be used in one approach as noted above, other approaches may also be used, and may actually be more advantageous. For example, multiple compression strokes may be used with port injection of fuel, along with open valve injection. Further, more than two compression strokes may also be used to achieve the desired evaporation of fuel, air and fuel mixing, and/or air/fuel ratio.